errors: add ERR_AMBIGUOUS_MODULE_SYNTAX message

lib/internal/errors.js

@@ -1133,7 +1133,12 @@
     return msg;
   },
   Error);
+
+  E('ERR_AMBIGUOUS_MODULE_SYNTAX',
+  'The module syntax is ambiguous. This file mixes CommonJS (require, module.exports) and ES Module features (import, export, or top-level await). Use either CommonJS or ES Modules consistently in the same file.',
+  Error);
+
 E('ERR_AMBIGUOUS_ARGUMENT', 'The "%s" argument is ambiguous. %s', TypeError);
 E('ERR_ARG_NOT_ITERABLE', '%s must be iterable', TypeError);
 E('ERR_ASSERTION', '%s', Error);
 E('ERR_ASYNC_CALLBACK', '%s must be a function', TypeError);

test/es-module/test-ambiguous-module-syntax.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { spawnSync } = require('child_process');
+const fixtures = require('../common/fixtures');
+
+// Skip if ESM is not supported in this environment
+common.skipIfESMUnsupported(__filename);
+
+// Fixture that mixes CJS + ESM
+const entry = fixtures.path('es-modules', 'ambiguous-mixed-syntax.js');
+
+const result = spawnSync(process.execPath, [entry], {
+  encoding: 'utf8'
+});
+
+// Node should exit with an error
+assert.notStrictEqual(result.status, 0);
+
+// Error code must be present
+assert.ok(
+  /ERR_AMBIGUOUS_MODULE_SYNTAX/.test(result.stderr),
+  `stderr did not contain ERR_AMBIGUOUS_MODULE_SYNTAX:\n${result.stderr}`
+);
+
+// Your new explanatory message should appear
+assert.ok(
+  /mixes CommonJS/.test(result.stderr),
+  `stderr did not contain expected message:\n${result.stderr}`
+);

test/fixtures/es-modules/ambiguous-mixed-syntax.js

@@ -0,0 +1,8 @@
+// This file intentionally mixes CommonJS and ES Module syntax
+// to trigger ERR_AMBIGUOUS_MODULE_SYNTAX.
+
+// CommonJS
+const fs = require('fs');
+
+// ES Modules
+export const value = 123;